Aerogel is a solid with a porous, sponge-like structure in which about 95% of the volume is empty space (that is, filled with air). Aero gels are therefore unique among solid materials. They possess extremely low densities, large open pores, and a large inner surface area. Discovered in the 1930s, aerogel is believed to be the world's lightest solid.
Aerogel thus possesses interesting physical and chemical properties including extremely low thermal conductivity, low sound velocity combined with excellent thermostability and mechanical properties. For silica aerogel, it also possesses high optical transparency. Currently, the aerogel materials are widely applied in the catalytic reactions, aerospace technology (storage, thickening or transport of rocket fuels and consisting of outside layer of spacecraft), advanced materials and so on. For example, Monsanto once produced aerogel granules made of silica for use as additives in cosmetics and toothpastes. NASA has used aerogels as insulation on the Mars Sojourner robot and as a medium to capture pure star dust, which was then returned to Earth for study.
Generally, the aerogels are prepared from molecular precursors via sol-gel method and following supercritical drying processing to exchange the pore liquid with air while maintaining the filigrane solid network. (Gesser, H. D.; Goswami, P. C. Chem. Rev., 89, (1989), 765.; Hüsing, N.; Schubert, U. Angew. Chem. Int. Ed., 37, (1998), 22.; Pierre, A. C.; Pajonk, G. M. Chem. Rev., 102, (2002), 4243.) By increasing the temperature and the pressure on the gel so that the liquid inside becomes “supercritical,” a special state in which a material has some properties of a liquid and some properties of a gas. Interestingly, supercritical liquid, like a gas, has practically no surface tension. The surface tension is believed to be the reason that causes collapse of the scaffolding made of the tiny network inside the gel as the liquid evaporates.
With the above basic approach, it is no longer difficult to make aerogels. Aerogels generally assume two physical forms: powder form (without defined shape) and monolithic form (with defined shapes). While it is relatively easy to make monolithic aerogels which are predominantly based on SiO2, there is no known easy procedure to make monolithic aerogels out of material that contains a substantial amount of non SiO2 components. In addition, aerogels tend to have a broad pore size distribution. Aerogels that are not purely made from SiO2 are useful for many applications, for example, for embedding catalysts therein for increasing catalytic efficiency due to aerogels' large surface-mass ratio wherein monolithic aerogels are advantageous than aerogel in powder form. The monolithic form prevents the nanoparticles from delamination, in particular, in the field of indoor air quality control by gas-phase photocatalytic oxidation reaction. Furthermore, catalysts embedded in monolithic aerogel help to overcome the practical drawbacks of the fine powders, including the difficult separation and handling for liquid-phase reactions or large pressure drops for gas phase reactions. Therefore, there is a great need for developing aerogel monolith not purely based on SiO2 as well as novel procedures to make such aerogel monolith easily and economically.